Corrosion Investigation of A516-Gr70 and API 5LX70 Steels in H2S Containing Solution

Caspian Journal of Applied Sciences Research, 1(11), pp. 1-10, 2012

Available online at http://www.cjasr.com

ISSN: 2251-9114, ©2012 CJASR

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Corrosion Investigation of A516-Gr70 and API 5LX70 Steels in H2S Containing

Solution

Saeid Kakooei1*, Hossein Taheri

2, Mokhtar Che Ismail

1, Abolghasem Dolati

3

1Center For Corrosion Research, Mechanical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri

Iskandar, 31750 Tronoh, Perak, Malaysia. 2Corrosion Engineering Group, Engineering Faculty, Kish University, Kish Island, Iran. 3Department of Materials Engineering, Sharif University of Technology, Tehran, Iran

*Corresponding Author: [email protected]

Environmental cracking in the presence of wet hydrogen sulfide (H2S) is a serious problem in the chemical or

petrochemical industries. This phenomenon causes major failure for pipeline steels transmitting sour gas/oil and

steel tanks storing liquefied petroleum gas (LPG). The effect of environmental parameters was studied for HSLA

steels (X70, A516) in sour simulated solution. The dissolved H2S was created by chemical reactions in solution.

Na2S·9H2O was chosen as test material to replace H2S because of the toxicity of H2S. The specimens were

immersed into synthetic seawater saturated with H2S where corrosion behavior was evaluated by

potentiodynamic polarization, weight loss, and scanning electron microscopy (SEM). The results showed that

the presence of alloying elements can change the corrosion rate in inspecting steels.

Key words: Hydrogen sulfide; Corrosion; Hydrogen induced cracking; Sour environment

1. INTRODUCTION

Oilfield systems experience pipeline failures due to

presence of aqueous H2S which cause aggressive

damage to the steels used in the transport and

processing of petroleum products. Inspection

studies on pipeline failures in the petroleum

refining industry indicate that 25% of failures are

associated by hydrogen diffusion. The wet H2S

reacts with steel will lead to generation of atomic

hydrogen. A part of atomic hydrogen will be

absorbed and penetrated into the steel(Carneiro et

al., 2003; Kittel et al., 2008). The diffused

hydrogen could be a reason of blistering or

hydrogen induced cracking (HIC) (Nasirpouri et

al., 2011).

Cathodic polarization of molecular surface

complex (Fe H-S-H) results in the release of

hydrogen atoms when aqueous H2S reacts with

steel. Some of the released hydrogen atoms will

diffuse into steel and others will recombine

(Elboujdaini et al., 2003). The mechanism is

shown below from equations.(1)-(4).

Fe + H2S + H2O →FeSH−

ads +H3O+

(1)

FeSH−

ads →FeSH+

ads +2e- (2)

FeSH+

ads →FeS + H+ (3)

FeSH+

ads + H3O+ →Fe

2+ +H2S +H2O (4)

Hydrogen diffuses shift stress gradients from

regions of lower to higher concentration. When the

concentration of hydrogen reaches a critical value

due to welding and cooling, the crack initiation

happens (Rogante et al., 2006). When the

concentration of hydrogen sulfide is low in a CO2

dominated system, it is reported that the iron

sulfide (FeS) film interferes with the formation of

the iron carbonate scale (FeCO3). Although, the

FeS film is believed to have a protective effect at

about 60°C (Brown et al., 2003).

Corrosion behaviors of two common pipeline

steels (X70 and A516) in different H2S

concentration brine solution are investigated in the

present study.

2. MATERIALS AND METHODS

Experiments were carried out at 50°C in a glass

cell (Figure 1). Two common pipeline steel A516-

Gr70 and API 5LX70 were investigated in this

study. Elemental composition of two inspected

steels is shown in Table 1. A typical three-

electrode setup was used with a saturated calomel

electrode (SCE) as the reference electrode, a

platinum counter electrode, and X70 and A516

steel specimens as the working electrodes. Steel

specimens were connected to copper wire and

covered with epoxy resin with an exposed area of

1cm2. The specimens were polished, degreased

with acetone and rinsed with distilled water before

conducting experiments.

mailto:[email protected]

Kakooei et al.

Corrosion Investigation of A516-Gr70 and API 5LX70 Steels in H2S Containing Solution

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Different concentrations of H2S containing

solution were made by using different

concentration of Na2S.9H2O , acetic acid, and

distilled water as shown in Table 2 (Taheri et al.,

2012). The base solution was 3% NaCl solution.

Fig. 1: Schematic of the experimental test cell: 1-platinum counter electrode 2- temperature probe 3-

reference electrode, 4-Chemical in, 5- sample holder (working electrode) 6-gas out.

Table 1: Elemental composition of X70 and A516 steels (in wt %).

Elements Chemical Composition

(wt%)

X70 A516

C 0.08 0.2

Si 0.29 0.3

Mn 1.59 1.05

P 0.013 0.035

S 0.002 0.04

Cu 0.08 --

Ni 0.1 --

Cr 0.02 --

Mo 0.11 --

V 0.047 --

Al 0.023 --

Nb 0.034 --

Ti 0.016 --

Fe Balance Balance


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Table 2: Different concentration of chemicals for test matrix

Material Concentration of Solution (mol/Lit)

C1 C2 C3 C4

Na2S.9H2O 0.015 0.035 0.055 0.075

CH3COOH 0.035 0.082 0.128 0.175

Corrosion rates in weight loss experiment were

measured by using the following equation [6]:

DAT

WRC

534.

(5)

Where C.R. is corrosion rate (mpy), ΔW is the

weight loss (mg), D is specimen density (g/cm3), A

is specimen exposure surface (in2), and T is

exposure time (hr).

The corrosion morphology of specimens and

hydrogen induce cracking were characterized by

SEM (Vega Tescan, USA). Corrosion products on

the corroded samples were analyzed using Energy

dispersive X-ray spectroscopy (EDAX).

3. RESULTS AND DISCUSSION

The potentiodynamic polarization curves for X70

and A516 steels under different H2S solution

concentrations at 50 °C are shown in Figures 2 and

3, respectively. The corrosion potential becomes

increasingly positive with increasing H2S

concentration. Two factors that influence the value

of the corrosion potential are the cathodic and

anodic processes. Although the individual

contribution of each process is not very significant,

a positive shift in the corrosion potential occurred

because the cathodic process on the metal surfaces

was promoted, and the anodic process was

retained. The combined effect of these processes

can be clearly observed in Figures 2 and 3.

However, Tang et al. (2010) showed that under a

high H2S concentration at 90 °C, the cathodic

hydrogen evolution process on a metal surface

increasingly influenced the cathodic branches with

increasing H2S concentration, thereby positively

shifting the potential. However, the anodic

branches remained nearly the same in all

solutions..

Fatah et al. (2011) indicated that a change in the

nature of the cathodic reaction in the presence of

S2-

ions is the main causative factor of the changes

in the cathodic reaction of both the Tafel slop and

redox potential, as shown in equations 5 and 6

(Fatah et al., 2011):

Na2S+H2O→2Na++HS

−+OH

− (5)

2HS−+2e

−→2S

2−(ads)+2H(ads) (6)

Figures 4 and 5 demonstrate the results of the

weight-loss experiment for the X70 and A516 steel

types, respectively. The variations in the corrosion

rate are due to the role of the FeS film in surface

passivation. The A516 steel samples dipped in

critical concentration (C2) exhibited a high

corrosion rate under different exposure times

because the FeS protective layer was detached

from the surface, thereby increasing the corrosion

rate.

For the X70 steel, by contrast, the protective

layer was apparently unstable under all

concentrations, and the average corrosion rate

increased with increasing H2S concentration. A

layer of the black corrosion film was detected on

the surfaces of all specimens after exposure to the

test solution.

The EDAX analysis of the corrosion product

indicates a high sulfur content due to the FeS film

on the sample surface (Figure 6). Table 1 shows the chemical composition of the

steel specimens. The Nb, Cu, and Mo present in

X70 steel produced carbon nitride during its

manufacture. Thus, the corrosion rate of this steel

is lesser than that of the A516 steel.Furthermore,

the susceptible area for the initiation of cracking in

the X70 steel is reduced. In contrast, the A516

steel is more susceptible to corrosion because of

the high MnS content, which is another causative

factor of the high corrosion rate of this steel.

Decreasing the sulfur and manganese contents can

decrease the corrosion rate of the A516 steel.

Kakooei et al.


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Fig. 2: Potentiodynamic polarization curves of X70 steel in the 3% NaCl solution with different concentration of H2S(C1, C2, C3, and C4) for different exposure times a) 24, b) 48,

c) 72, d) 96 hr at 50ºC


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Fig. 3: Potentiodynamic polarization curves of A516 steel in the 3% NaCl solution with different concentration of H2S(C1, C2, C3, and C4) for different exposure times a) 24, b) 48,

c) 72, d) 96 hr at 50ºC .

Kakooei et al.


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Fig. 4: 3D column chart of corrosion rate of X70 steel vs. different H2S concentration C1, C2, C3, and C4 with different exposure time at 50ºC.

Fig. 5: 3D column chart of corrosion rate of A516 steel vs. different H2S concentration C1, C2, C3, and C4 with different exposure time at 50ºC.


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Fig. 6: EDAX analysis of corrosion product after removing form steel surfaces

Kakooei et al.


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Figures 7 and 8 respectively illustrate the SEM

images of the X70 and A516 steel samples after

exposure to H2S for 96 h at 50 °C. The cracks in

the FeS film demonstrate that this layer is unstable

for surface passivation in X70 steel and cannot

protect the steel surface from further corrosion

(Figure 8). By contrast, the FeS film on the A516

steel surface is stable and uniform; hence, it can

protect this steel from further corrosion. Metallographic cross sections of the hydrogen-

induced cracking in both steel types are shown in

Figures 9 and 10. The color mapping of the

corrosion product on the A516 steel surface

emphasizes the presence of FeS on the specimen

surface (Figure 11).

Fig. 7: SEM image of corrosion product on A516 steel surface

Fig. 8: SEM image of corrosion product on X70 steel surface


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Fig. 9: Metallographic cross section of A516 steel at 50°C

Fig.10: Metallographic cross section of X70 steel at 50°C

Fig. 11: Color mapping of corrosion product on steel surface

4. CONCLUSION

The FeS film was detected on the surfaces of both

steel types, although with different morphologies

and stabilities. The result shows that hydrogen-

induced cracking can occur in solutions with H2S

for the X70 and A516 steel types. The width of the

crack in the A516 steel was greater than that in the

X70 steel, indicating that A516 steel is more

susceptible to HIC . However, the presence of

alloying components, such as Cu and Nb, can

reduce the occurrence of HIC in X70 steel. The

Kakooei et al.


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average corrosion rates of the X70 specimens were

higher than those of the A516 specimens. The

corrosion rates of X70 steel did not show

considerable changes under the different H2S

concentrations, whereas A516 exhibited a C2 value

that enhanced the corrosion rate by twice the initial

rate. This condition indicates the unstability of the

FeS film under C2 concentration.

Acknowledgments

Facilities and funding for this study were provided

by Kish University, Iran. Also authors would like

to thank Universiti Teknologi PETRONAS for

supporting the research work.

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Documents

Corrosion Investigation of A516-Gr70 and API 5LX70 Steels in H2S Containing Solution